Resolution of racemic aryl-substituted aliphatic carboxylic acids has been described in the literature. Kaiser et al., J. Pharm. Sci., Vol. 65, No. 2, 269-273 (February 1976) formed the S(-) .alpha.-methylbenzylamine salt of S(+)-ibuprofen, removed it from the reaction mixture by filtration, and recrystallized it from isopropanol and then from methanol. After acidifying the 3N aqueous sulfuric acid and extracting with ether, S(+)-ibuprofen was obtained, m.p. 50.14 52., [.alpha.].sub.D +57., with 95% optical purity as determined by GLC analysis. Cox et al., J. Pharmacol. Exp. Ther., Vol. 232, No. 3, 636-643 (March 1985), using the Kaiser et al. method, were able to obtain an S(+)-ibuprofen preparation which was 99% S isomer and 1% R isomer (w/w).
Other methods of separating the enantiomers of racemates can be effected by preparing a salt of the acid with an alkaloid or similar resolving agent such as cinchonidine, then separating the products by fractional crystallization from a solvent in which the salt of the dextrorotatory isomer is less soluble. The (+)-salt can then be acid cleaved to yield pure enantiomer. See, for example, U.S. Pat. No. 4,209,638 issued Jun. 24, 1980, and U.S. Pat. No. 3,637,767 issued Jan. 25, 1972, which relate to resolution of naproxen and related compounds.
U.S. Pat. No. 5,015,764 discloses and claims a process for increasing the amount of the desired enantiomer obtained from a racemic mixture of C.sub.1 to C.sub.6 linear or branched aliphatic carboxylic acid or ester thereof. The process comprises: (i) forming a salt solution comprising the racemic mixture of the C.sub.1 to C.sub.6 linear or branched aliphatic carboxylic acid or ester thereof and an organic or inorganic base; (ii) treating said salt solution with a chiral organic nitrogenous base having a base strength no stronger than said organic base, inorganic base or mixtures of an organic base and an inorganic base; (iii) precipitating from the reaction solution produced in the treatment of step (ii) the less soluble diastereomeric salt; and (iv) separating said diastereomeric salt. The disclosure of this patent is incorporated herein by reference.
According to the process of the present invention, an improvement of the above process has been discovered. Reaction steps (i), (ii) and (iii) are carried out as disclosed. At this point in the reaction sequence, a two-phase mixture is produced that is essentially the solid diastereomeric salt and the remaining reaction liquid. The solid is dispersed in near emulsion form throughout the liquid. It is typically separated by filtration leaving the mother liquor filtrate and solid filtered residue. The residue requires numerous recrystallizations before a product of satisfactory purity is obtained. The conventional separation processes are inconvenient and time consuming, disadvantageously producing multiple process streams.
It has now been discovered that an improved crystalline product can be obtained from the mixture of step (iii) by adding an inert liquid having a different density than the density of the reaction mixture. Surprisingly, when the less dense, inert liquid is added and mixed into the reaction mass, when the mixing action is stopped, the solid phase readily separates from both the reaction solvent and the inert liquid. A three-phase mixture typically results, each layer being easily separated from the other by simple mechanical means (decantation and the like). The inert liquid, if less dense than any of the other components of the mixture, usually forms the uppermost layer; if more dense than the other components, usually forms the lowermost layer.
The inert liquid, however, must have appreciable ability to solubilize one of the diastereomeric salts, preferentially more than the other diastereomeric salt. As such, a solubility of 1 gram of salt per cubic centimeter of inert liquid produces an acceptable inert liquid. This characteristic can be readily identified when, after adding a potential inert liquid and mixing, a phase separation occurs. Further, it should be substantially immiscible with the reaction solution. If either of these conditions should occur, then the density of the liquid will be affected and the ability to cause the phase separation will be lost.
Since the process can be carried out in either aqueous or hydrocarbon medium, the inert liquid can be either a hydrocarbon, water, formamide, acetamide, N,N-dialkyl, substituted formamide or acetamide, as long as the above criteria are met. Thus, when reaction steps (i), (ii) and (iii) occur in water, formamide, acetamide, substituted formamide or acetamide as the reaction medium, the inert liquid is an aliphatic or aromatic hydrocarbon optionally substituted with one or more halo (chloro or bromo), nitro, amino, cyano, carboxylic acid or C.sub.1 to C.sub.6 linear or branched alkyl ester thereof, hydroxy, thio, thioether--the substituent or the sulfur being C.sub.1 to C.sub.6 linear or branched alkyl, or C.sub.1 to C.sub.6 linear or branched alkyl, and the reverse is also true.
Preferably, under these reaction conditions, the inert liquid is a C.sub.5 to C.sub.12 linear or branched hydrocarbon optionally substituted with one or more halo groups or it is an aromatic hydrocarbon optionally substituted with one or more C.sub.1 to C.sub.6 linear or branched alkyl or halo group. Most preferably, the inert liquid is selected from the group consisting essentially of hexane, heptane, octane, benzene, toluene, xylene or mixtures thereof.
When the reaction medium is a hydrocarbon one, water is preferably used as the inert liquid.
The C.sub.1 to C.sub.6 linear or branched aliphatic carboxylic acids and esters useful in the improved process of the present invention have the formula ##STR1## where R.sub.1 is hydrogen or C.sub.1 to C.sub.6 linear or branched alkyl; R.sub.2, R.sub.3 and R.sub.4 are independently the same or different and are hydrogen or C.sub.1 to C.sub.6 linear or branched alkyl, e.g., methyl or ethyl; aralkyl, e.g., benzyl; C.sub.3 to C.sub.6 cycloalkyl, e.g., cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl; alkyl substituted cycloalkyl, e.g., methylcyclohexyl; C.sub.6 to C.sub.10 aryl, e.g., phenyl unsubstituted or substituted with one or more, for example, methyl, dimethyl, butyl, especially isobutyl or phenyl substituted with one or more C.sub.1 to C.sub.4 alkylthio, C.sub.1 to C.sub.4 alkoxy, cyano or halo, e.g., fluoro or chloro; C.sub.1 to C.sub.6 linear or branched aryloxy, e.g., phenoxy or phenoxy substituted with, for example, methyl, dimethyl, butyl or isobutyl or phenoxy substituted with C.sub.1 to C.sub.4 alkylthio, C.sub.1 to C.sub.4 alkoxy, cyano or halo; C.sub.1 to C.sub.6 alkylthio, e.g., methylthio; C.sub.2 to C.sub.8 cycloalkylthio; C.sub.6 to C.sub.10 arylthio; C.sub.6 to C.sub.10 arylcarbonyl, e.g., benzoyl; C.sub.4 to C.sub.8 cycloalkenyl, e.g., cyclohexenyl; trifluoromethyl; halo, e.g., fluoro or chloro; C.sub.4 to C.sub.5 heteroaryl, e.g., furyl, pyrrolyl, thienyl; or C.sub.10 to C.sub.14 aryl, e.g., naphthyl or naphthyl substituted with C.sub.1 to C.sub.4 alkyl, e.g., methyl, C.sub.1 to C.sub.4 alkoxy, e.g., ethoxy, halo; or biphenyl unsubstituted or substituted with methyl or halo, especially fluoro.
Preferred compounds of formula I are those of the formula ##STR2## where R.sub.1, R.sub.2 and R.sub.3 are as previously defined and R.sub.5 and R.sub.6 are C.sub.1 to C.sub.4 linear or branched alkyl, C.sub.1 to C.sub.4 linear or branched alkoxy or halo.
The improved process is particularly applicable to 2-(4-isobutylphenyl)propionic acid and especially in obtaining a preponderance of the S(+) isomer.
The process is carried out by using a racemic mixture [a mixture of both the (+) and (-) or dextro and levo rotatory forms] or a mixture containing a preponderance of one of the enantiomers of these carboxylic acids. However, it should be understood that in this step, the process itself does not convert one form of the stereoisomers to the other form but only separates such forms. Further, because the separation of isomers gives rise to a soluble product largely containing one enantiomer and an insoluble product largely containing the other enantiomer, a high purity salt is obtained that requires a minimum number of recrystallizations (usually not more than two) to give a product with exceptional high optical purity.
The purified salt obtained from the process of the present invention may be further treated to produce the free aliphatic carboxylic acid thereof by using any conventional means. For example, hydrolysis of the salt an acid and extraction with a suitable solvent produces the purified aliphatic carboxylic acid. Further extraction and recrystallization with a suitable solvent can increase the purity to even greater extent.
The first step in the reaction sequence for the separation of the racemic mixtures used in the present invention is to form a salt solution of the aliphatic carboxylic acid with an organic or inorganic base. Where such organic base is used in this first step, the solvent employed to form the salt solution is preferably an inert liquid. Most preferably, such solvents include the aliphatic hydrocarbon solvents, C.sub.4 to C.sub.14 hydrocarbons, formamide, acetamide, N,N-dialkyl (C.sub.1 to C.sub.6), substituted formamides or acetamides, e.g., compounds of the formula R.sub.1 C(R.sub.2)HC(O)NH.sub.2 where R.sub.1 and R.sub.2 are the same or different and are C.sub.1 to C.sub.6 linear or branched alkyl, or water. Particularly preferred is hexane, octane or water as such solvent.
The chiral organic nitrogenous base is next added in less than equimolar quantity. It forms a more stable salt with the isomers of the aliphatic carboxylic acid displacing the inorganic or organic base. Further, one of the diastereomeric salts formed from the subsequent displacement of the inorganic or organic base by the chiral organic nitrogenous base is more soluble in the reaction solution (the solution formed when the chiral base is added to the salt solution), the other, of course, precipitates. The solid precipitate is separated from the solution by conventional techniques, i.e., centrifugation, filtration and the like.
The next step in the process is to add the new solvent to extract the unreacted carboxylic acid salt made with the organic or inorganic base. This solvent, referred to as the countersolvent, separates the carboxylic acid chiral base salt from the carboxylic acid organic base salt. If the reaction sequence is conducted in a hydrocarbon solvent, the countersolvent is water, amides, etc. If the reaction sequence is conducted in water, formamide, and the like solvents, the countersolvent is a hydrocarbon solvent.
It should be noted that the process of the present invention achieves the same end result upon change in sequence of addition of the solvents, i.e., the solvent followed by the countersolvent addition, the countersolvent followed by the solvent addition, or the simultaneous addition of solvent and countersolvent perform equally as well.
It should be noted that the process of the present invention is particularly adapted to the economical conversion of mixtures to the diastereomeric S- or (+)- component. (Of course, the R-component may be the least soluble one, in which case the following discussion should be applied in reverse). The method of the present invention essentially provides a solid precipitate enriched in the S-enantiomer suspended in one solvent and a liquid filtrate enriched in the R-enantiomer in another solvent. Liberation of the desired S-enantiomer from the precipitated salt suspended in one solvent is readily accomplished by acidification of the salt with, for example, dilute mineral acid or any other inorganic or organic acid conventionally known to hydrolyze salts of this nature. While this procedure leaves the filtrate as a by-product, it can be further treated with acid or base to convert the R-enriched filtrate to the racemic mixture. This mixture can then be reused in the process of the present invention, using the chiral organic base recovered from the above conversion step. Thus, the process of the present invention lends itself readily to a recycling-type of procedure.
While the above reactions are carried out in a mixture of water and triethylamine, it has been discovered that the aryl-substituted profens (ibuprofen, ketoprofen, etc.) are surprisingly soluble in solvent mixtures of tri C.sub.1 to C.sub.6 linear or branched aliphatic amines and water (i.e., from 1% amine up to 50% amine). However, when using aryl or aralkyl tertiary amines (such as methylbenzyl amine), such profens display limited or no solubility in mixed water-containing solvent systems. Therefore, these aliphatic amines/water systems can be used to recrystallize these profens.